1. Field of the Disclosure
The present invention relates to the preparation for separating workpieces and substrates using ultra-short pulsed laser radiation. In particular toughened glass or glass ceramics are contemplated as a workpiece material. The invention also relates to the separation of workpieces.
2. Description of Related Art
From WO 2012/006736 A2 it is known that the Kerr effect can be exploited to cause irreversible damages in glass in form of filaments. By generating a linear array of such damages in glass it is possible to separate transparent substrates. A filament is formed by an ultra-short laser pulse. Due to the Kerr effect, the laser beam experiences self-focusing in the interior of the glass until the energy density at a point becomes so high that a plasma is ignited. A plasma explosion is caused during which the glass undergoes irreversible damage around this plasma generation location. From there further radiation emanates which is subject to self-focusing and ends up in a plasma explosion. This effect is repeated several times, depending on the intensity. Energy decreases along the entire thickness of the glass, so the first plasma spots will have the highest energy and produce the greatest damages. Furthermore, the plasma spots are round, which means that emanating defects will occur randomly distributed in all directions.
In glass that exhibits introduced stresses, for example, chemically toughened glass, spontaneous self-breakage may occur whereby the processing of especially comparatively large glass sheets is considerably affected. As a result of breakage the position of the glass sheet changes. Further exact processing is impossible.
Patent document DE 102 13 044 B3 describes a method for cutting or drilling material. Here, again, the nonlinear optical effect occurring with high-intensity ultra-short laser pulses is exploited to produce a filament due to the alternating focusing and defocusing of the laser beam. Accordingly, a filament is a passage of small diameter produced by a high-intensity laser light beam.
Furthermore, document DE 10 2006 042 280 A1 describes a method for processing transparent material using a laser. Ultra-short laser pulses are used to generate both a surface groove on the substrate and one or more laser-modified regions in the volume of the material. The fracture ultimately leading to the separation occurs at the superficial scribing trace and propagates downwards across the substrate material. If the surface groove is too flat, the fracture tends to migrate. A generation of breaking edges with consistent high quality is not described.
DE 10 2007 028 042 B2 also discloses a method for laser processing of transparent materials and describes a use of pulsed laser radiation in the nanosecond range. The document mentions a range of radiation intensity in which material changes occur without plasma luminescence.
In summary, various processes have been known which allow to modify regions in the volume of a material by means of ultra-short pulsed laser radiation so as to provide one step of a separation process. However, the separating and breaking which is required for example for dicing substrates that have been modified in this manner, has hitherto not been sufficiently accessible to industrial processes. This problem is particularly acute with substrates comprising toughened glass or glass ceramics, as these are prone to uncontrolled breakage due to inherent stresses introduced by the toughening, when processed with ultra-short pulsed laser radiation.
However, for industrial application exact control is not only required for the generation of a separation line in or on the substrate, but also for the separating or breaking in order to produce breaking edges of consistent high quality and to ensure stability and safety of the process. This is very difficult particularly in case of toughened glass, since the material modifications caused by the laser irradiation can lead to an uncontrolled occurrence and propagation of cracks, so that accurate control of separation is very difficult.
The following issues are of concern: Cutting/Drilling using filamentation: due to the process, formation of the filament occurs inhomogeneously: due to the high initial energy density, comparatively larger plasma volumes are ignited on the entry side of the filament producing laser beam than at the subsequent plasma spots deeper in the workpiece, i.e. the channel of damages in the workpiece (corresponding to the filament formed) will taper. The induced damages (microcracks) will thus be much stronger on the entry side of the laser beam than on the exit side. Directional strength tests (four-point bending) reveal a significant difference in edge strength already with a glass thickness of 0.7 mm. Spatial geometry of the plasma generation spots: The plasma generation spots caused by self-focusing have a substantially spherically symmetrical shape with spherically symmetrical energy distribution, which causes direction-independent randomly distributed microcracks around the plasma volume. As a result, cracks will even protrude into the later breaking edge and have a strength-reducing effect. Spontaneous breakage: During filamentation of brittle materials with intrinsic stresses, uncontrolled spontaneous breakage of the workpiece occurs during the process, resulting in an increased rejection rate. Furthermore, spontaneous breakage causes a change in the position of the workpiece, so that automated processing is impeded or even made impossible.